package/lftp: add optional support for expat

package/lftp: add optional support for expat

When expat was compiled before, the proto-http module will use it as
optional dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/lftp/4.6.4/proto-http.so | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libcrypto.so.1.0.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libexpat.so.1]
[...]

The build system offers no option to en-/disable expat support:
https://github.com/lavv17/lftp/blob/lftp-4.6-stable/configure.ac#L337

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>